Conventional articles of footwear include two primary elements, an upper and a sole structure. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure is secured to a lower surface of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces, the sole structure may provide traction and control foot motion, such as over pronation. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a wide variety of ambulatory activities, such as walking and running.
The upper forms a void on the interior of the footwear for receiving the foot. The void has the general shape of the foot, and access to the void is provided by an ankle opening. Accordingly, the upper extends over the instep and toe areas of the foot, along the medial and lateral sides of the foot, and around the heel area of the foot. This general structure for the upper may vary depending upon the type of footwear. For example, the upper of a sandal may only cover a portion of the foot, and the upper of a boot may extend upward to cover a portion of the ankle. Various materials may be utilized in manufacturing the upper. The upper of an article of athletic footwear, for example, may be formed from multiple material layers that include an exterior layer, a middle layer, and an interior layer that are formed from one or more of leather, synthetic leather, textiles, polymer sheets, mesh materials, and foam, for example. In order to join the layers and form the upper, adhesives, stitching, or a combination of adhesives and stitching may be utilized.
The sole structure generally incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin, cushioning member located within the upper and adjacent the plantar (lower) surface of the foot to enhance footwear comfort. The midsole, which may be adhesively-secured or stitched to the upper, forms the middle layer of the sole structure and serves a variety of purposes that include attenuating ground reaction forces and controlling foot motions. The outsole is bonded to the midsole and forms a ground-contacting element of the footwear. The insole may be formed from textiles, polymer foam, or a combination of textiles and foam, for example. The primary material for a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate. In some articles of footwear, the polymer foam material may encapsulate a fluid-filled bladder or other elements. The outsole is generally formed from a rubber material with a relatively high degree of wear resistance.
Based upon the above discussion, conventional articles of footwear are formed from a variety of materials that are permanently joined with adhesives or stitching, for example. When one footwear element (e.g., upper, sole structure, midsole, outsole) becomes worn or otherwise damaged, the permanent nature of the adhesive or stitching effectively limits the ability to replace the footwear element. Accordingly, the footwear may be unusable if even one footwear element is damaged. Similarly, the variety of materials forming the various footwear elements, coupled with the permanent nature of the adhesive or stitching, effectively limits the ability to recycle the various footwear elements. Upon the expiration of the useful life of the footwear, therefore, the footwear or individual footwear elements may not generally be recycled because the various footwear elements may not be easily separated.